In the recent progress of electronics, the impartation of electric conductivity to the surface of plastics has become a particularly important theme. With such plastics advancements have occurred in, e.g., static prevention for preventing the occurrence of various problems caused by static electricity, for example, the occurrence of the attachment of dust, etc., electric discharging caused by electrostatic charging, and electromagnetic wave obstruction of casings and parts of electronic instruments.
Transparent electroconductive films have been widely used, e.g., as base materials for electrophotographic recording, base materials for electrostatic photographic recording, transparent electrodes for thin film type liquid crystal displays, transparent electrodes for dispersion type electroluminescence, transparent electrodes for touch panels, antistatic films for clean rooms, windows of electric meters, video tape recorders, etc., transparent heaters, etc. The development of transparent electroconductive films which are inexpensive and have high performance has been strongly desired.
Conventional transparent electroconductive films include the semiconductor type thin films, such as indium tin oxide films (ITO films) doped with tin, tin oxide films doped with antimony, cadmium tin oxide films (CTO films), copper iodide films, titanium oxide films, and zirconium oxide films. Among these films, the ITO films are most excellent in terms of both transparency and electric conductivity. Tin oxide films require a high base plate temperature for forming films and hence it is difficult to apply such a film to a polymer film. CTO films have a smaller energy gap (the absorption end is at a longer wavelength side) than the ITO films. Thus, when the film thickness is increased, the film become yellowish to some extent. Also, copper iodide films, titanium oxide films, and zirconium oxide films are inferior in both transparency and electric conductivity to the aforesaid semiconductor films.
The above semiconductor thin electroconductive films are formed by, e.g., vapor deposition which requires large production equipment for forming the films, which increases the production cost.
As a method of forming the above semiconductor thin films at a low cost, a method of previously applying a subbing layer to a polymer film and letting a compound semiconductor absorb in the surface of the subbing layer is known. According to this method, the subbing layer can improve the adhesion of the support for a layer further formed thereon as described in JP-B-48-9984 (corresponding to U.S. Pat. No. 3,597,272) (the term "JP-B" as used herein refers to an "examined Japanese patent publication").
Hitherto, for a coating type electroconductive film using a compound semiconductor is formed by a method of forming a subbing layer on a support using a resin having adhesivity to the support and coating thereon a solution of a compound semiconductor to form fine particles of the compound semiconductor near the surface of the subbing layer at a high concentration.
However, although the electroconductive film of semiconductor formed by the aforesaid method is, in the beginning, excellent in terms of adhesion to the support, the transparency, and the electric conductivity, there are disadvantages in that the fine particles of the compound semiconductor become aggregated over the passage of time so as to form large crystals. This causes white turbidity and reduces the transparency. Further, the electric conductivity is greatly reduced at the white turbid portions.
When a commercially available vinylidene chloride resin or vinyl chloride resin coating material is used as the binder for the subbing layer, the components formed by the photodecomposition, etc., reduce the electric conductivity of the compound semiconductor such that is not useful for practical purposes in a field requiring light fastness.
In the wide application field of a transparent electroconductive films, it is as a matter of course required that the electric conductivity be stable for a long period of time. Depending on the usage, it is also important that the film has a resistance to organic solvent solubility.
For example, in the case of applying the transparent electroconductive film to an electrophotographic recording material, the electroconductive film is used in a form of a multilayer structure formed by coating a barrier layer, a layer of a photoconductive composition, a protective layer, etc., on an electroconductive film.
In the case of forming these multilayer coating structures, it frequently happens that the coating solvent causes fine cracks or creases in or on the subbing layer and the electroconductive layer, which gives serious problems for practical use. Hence, it has been desired to solve these problems.
Furthermore, the adhesion of such a coating layer and the electroconductive layer of a compound semiconductor is frequently insufficient. Hence, improvement of the adhesion has also been desired.